5 ms·

If you're referring to the Everett multiverse (although the OP was not, so maybe you're not), then its status is not so different from that of the unobservable

If you're referring to the Everett multiverse (although the OP was not, so maybe you're not), then its status is not so different from that of the unobservable portion of our own universe. That is, if we take the simplest known cosmological model consistent with our evidence, its equations predict space and matter outside our observable universe. That cosmological model is falsifiable in that it makes other predictions besides this one, many of which can be (and have been) tested, but this specific prediction cannot be directly tested. Similarly, the equations of quantum mechanics predict multiple "worlds" (although I find this term unfortunate: the state of the "multiverse" is always just a single point in Hilbert space, and it only seems like there are multiple "worlds" when we imperfectly try to map that physical state onto the intuitive level of everyday experience). That specific prediction cannot be directly tested, but other predictions of the equations can be, and have been. There is essentially no dispute about the math. The dispute is whether to deny the reality of something which is right there in that math, akin to denying the reality of a universe beyond that which is observable.

simiones 6y ago
The multiverse usually refers to multiple universes each with their own big bang, existing in regular space, but outside our light cone. It is a completely unscientific theory, in the sense that there is no chance of ever proving or denying it, by its very construction. The many worlds interpretation is completely unrelated, and it doesn't suffer from this problem of being unfalsifiable even in principle. However, I would also not say that the MWI is a straightforward deduction from the laws of QM. It rejects the most visible effect of those laws (the Born rule, i.e. that they predict probabilities of classical events and not certainties) and replaces it with an infinity of "worlds". Even worse, it still doesn't explain WHY we can only "see" one such "world", while elementary particles can "see" all of them (i.e. it doesn't solve the measurement problem any more than Copenhagen interpretation). And there is also debate on how plausible it is to even define the "probability of a world", so whether MWI is actually consistent with the Born rule is somewhat in doubt.
- DavidSJ 6y ago
  it still doesn't explain WHY we can only "see" one such "world" It's just that there's one of us in each world. The one in this world sees this one. The one in that world sees that one.
  - TheOtherHobbes 6y ago
    Define "sees." That's just one of the many problems that MWI fails to solve.
    - ummonk 6y ago
      The local wavefunction part corresponding to each world has a configuration in it that involves a human being with eyes and optical nerves and a brain registering neural impulses in response to light stimuli to perceive its world.
    - DavidSJ 6y ago
      In MWI, the problem doesn't exist to begin with. In e.g. the Copenhagen interpretation, observation plays a special role, because it's the moment of wave function collapse. In MWI, there is no wave function collapse, so there's no need to identify certain special events called "observations" which are somehow different from the rest of physics. It's just our eyes, and brains, and instruments, interacting and becoming entangled with the other things in the world. When we "observe" Schrödinger's cat, we entangle ourselves with the cat just as the cat was already entangled with the cesium atom. Now the world is in a superposition of two states: in one, the cat is alive and we see it as alive; in the other, the cat is dead and we see it as dead. This is exactly what the equations say should happen. Nothing special.
      - simiones 6y ago
        And yet, photons can keep interacting with both versions, while we are forever unable to see the other version of the cat. The problem doesn't go away. Even more interestingly, say you want to predict the outcome of a quantum experiment. In the MWI, that outcome is deterministic, and it is a superposition in Hilbert space. However, in your own space, you still need to compute a probability for that outcome. What is this a probability of in the MWI? The concept of probability across worlds is not really rigorously defined, especially since you need to arrive at very specific results to be consistent with observations.
boublepop 6y ago
> Similarly, the equations of quantum mechanics predict multiple "worlds" No they don’t. If I roll a dice it’ll land on one of 6 faces. That’s a model, in that model I might assume equal probability due to the geometrical symmetry, that’s an assumption. So now I have a problem if all outcomes are equally likely how do I resolve that only one occurs? Well obviously every dice roll must spawn 6 parallel universes... Now nowhere in that model or that assumption does “so there must be 6 parallel worlds spawned when you roll the dice” get predicted by my model. The multiverse/many world is an interpretation, not an outcome of, or even prediction from the model. There is nothing “right there in the math”. QM makes predictions about probabilistic outcomes, accurate predictions, but there is no requirement for those accurate predictions to be right that there must be multiple parallel worlds.
- simiones 6y ago
  > There is nothing “right there in the math”. QM makes predictions about probabilistic outcomes, accurate predictions, but there is no requirement for those accurate predictions to be right that there must be multiple parallel worlds. While I mostly agree with you, the discussion is much more complex than this. First of all, most of QM is completely deterministic, just like classical and relativistic mechanics. The Schrodinger equation is a linear partial differential equation, it gives you an exact prediction of the state of any system of particles. However, that prediction turns out to be so wrong on the face of it, it's almost meaningless: the Schrodinger equation predicts that "particles" (wave-packets) have some complex amplitudes of many different mutually-exclusive states, such as being here with amplitude 1+i and being there with amplitude 1-i. BUT, it turns out that if we interpret these amplitudes of the wave function (well, the squares of their absolute values) as being probabilities of the particle being in those states when measured, then you get extremely accurate predictions. The final problem then is that the amplitudes -> probabilities step is only correct when predicting the result of a measurement. At the particle level (before the measurement) the particles actually exist in all those states at the same time, and interact with each other in all those states (e.g. particle A may interact with particles B and C at the same time in two different places, modifying both their trajectories). So, the laws of motion for QM that you need to use are: 1. Particles move and interact according to the Schrodinger equation (e.g. causing interference patterns, interacting in multiple places at the same time), fully deterministically and linearly 2. When you want to predict the outcome of a measurement on a system that has evolved according to 1, you will get a single value with some probability computed as the square of the absolute values of the amplitudes of the possible states. After the measurement, the amplitude of any other state than the one measured will be 0, and this can be plugged in to make further predictions about how it will interact with other particles. It is this 2nd postulate that feels very artificial, the main subject of the measurement problem. MWI seeks to explain QM without the 2nd postulate - that is why people say that it "derives from the math" (the 2nd postulate can obviously not be derived from the first, as it is a nonlinear change to the system).

simiones 6y ago

The multiverse usually refers to multiple universes each with their own big bang, existing in regular space, but outside our light cone. It is a completely unscientific theory, in the sense that there is no chance of ever proving or denying it, by its very construction. The many worlds interpretation is completely unrelated, and it doesn't suffer from this problem of being unfalsifiable even in principle. However, I would also not say that the MWI is a straightforward deduction from the laws of QM. It rejects the most visible effect of those laws (the Born rule, i.e. that they predict probabilities of classical events and not certainties) and replaces it with an infinity of "worlds". Even worse, it still doesn't explain WHY we can only "see" one such "world", while elementary particles can "see" all of them (i.e. it doesn't solve the measurement problem any more than Copenhagen interpretation). And there is also debate on how plausible it is to even define the "probability of a world", so whether MWI is actually consistent with the Born rule is somewhat in doubt.

DavidSJ 6y ago
it still doesn't explain WHY we can only "see" one such "world" It's just that there's one of us in each world. The one in this world sees this one. The one in that world sees that one.
- TheOtherHobbes 6y ago
  Define "sees." That's just one of the many problems that MWI fails to solve.
  - ummonk 6y ago
    The local wavefunction part corresponding to each world has a configuration in it that involves a human being with eyes and optical nerves and a brain registering neural impulses in response to light stimuli to perceive its world.
  - DavidSJ 6y ago
    In MWI, the problem doesn't exist to begin with. In e.g. the Copenhagen interpretation, observation plays a special role, because it's the moment of wave function collapse. In MWI, there is no wave function collapse, so there's no need to identify certain special events called "observations" which are somehow different from the rest of physics. It's just our eyes, and brains, and instruments, interacting and becoming entangled with the other things in the world. When we "observe" Schrödinger's cat, we entangle ourselves with the cat just as the cat was already entangled with the cesium atom. Now the world is in a superposition of two states: in one, the cat is alive and we see it as alive; in the other, the cat is dead and we see it as dead. This is exactly what the equations say should happen. Nothing special.
    - simiones 6y ago
      And yet, photons can keep interacting with both versions, while we are forever unable to see the other version of the cat. The problem doesn't go away. Even more interestingly, say you want to predict the outcome of a quantum experiment. In the MWI, that outcome is deterministic, and it is a superposition in Hilbert space. However, in your own space, you still need to compute a probability for that outcome. What is this a probability of in the MWI? The concept of probability across worlds is not really rigorously defined, especially since you need to arrive at very specific results to be consistent with observations.

boublepop 6y ago

> Similarly, the equations of quantum mechanics predict multiple "worlds" No they don’t. If I roll a dice it’ll land on one of 6 faces. That’s a model, in that model I might assume equal probability due to the geometrical symmetry, that’s an assumption. So now I have a problem if all outcomes are equally likely how do I resolve that only one occurs? Well obviously every dice roll must spawn 6 parallel universes... Now nowhere in that model or that assumption does “so there must be 6 parallel worlds spawned when you roll the dice” get predicted by my model. The multiverse/many world is an interpretation, not an outcome of, or even prediction from the model. There is nothing “right there in the math”. QM makes predictions about probabilistic outcomes, accurate predictions, but there is no requirement for those accurate predictions to be right that there must be multiple parallel worlds.

simiones 6y ago
> There is nothing “right there in the math”. QM makes predictions about probabilistic outcomes, accurate predictions, but there is no requirement for those accurate predictions to be right that there must be multiple parallel worlds. While I mostly agree with you, the discussion is much more complex than this. First of all, most of QM is completely deterministic, just like classical and relativistic mechanics. The Schrodinger equation is a linear partial differential equation, it gives you an exact prediction of the state of any system of particles. However, that prediction turns out to be so wrong on the face of it, it's almost meaningless: the Schrodinger equation predicts that "particles" (wave-packets) have some complex amplitudes of many different mutually-exclusive states, such as being here with amplitude 1+i and being there with amplitude 1-i. BUT, it turns out that if we interpret these amplitudes of the wave function (well, the squares of their absolute values) as being probabilities of the particle being in those states when measured, then you get extremely accurate predictions. The final problem then is that the amplitudes -> probabilities step is only correct when predicting the result of a measurement. At the particle level (before the measurement) the particles actually exist in all those states at the same time, and interact with each other in all those states (e.g. particle A may interact with particles B and C at the same time in two different places, modifying both their trajectories). So, the laws of motion for QM that you need to use are: 1. Particles move and interact according to the Schrodinger equation (e.g. causing interference patterns, interacting in multiple places at the same time), fully deterministically and linearly 2. When you want to predict the outcome of a measurement on a system that has evolved according to 1, you will get a single value with some probability computed as the square of the absolute values of the amplitudes of the possible states. After the measurement, the amplitude of any other state than the one measured will be 0, and this can be plugged in to make further predictions about how it will interact with other particles. It is this 2nd postulate that feels very artificial, the main subject of the measurement problem. MWI seeks to explain QM without the 2nd postulate - that is why people say that it "derives from the math" (the 2nd postulate can obviously not be derived from the first, as it is a nonlinear change to the system).